The read and write head portions of the magnetic transducer for use in a typical prior art magnetic disk recording system are built-up in layers using thin film processing techniques. Typically the read head is formed first, but the write head can also be fabricated first. The conventional write head is inductive and the read sensor is magnetoresistive. In the typical process of fabricating thin film magnetic heads, a large number of heads are formed simultaneously on a wafer. After the basic structures are formed the wafer is cut into rows or individual devices which are also called sliders.
CMP is used at various stages in the fabrication of thin film magnetic heads for photoresist lift-off and to planarize the wafer. One problem in CMP operations is determining when the polishing process is complete. If the CMP continues longer than necessary, then damage to the components can result. Variations in the thickness of the layers, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and pressure can cause variations in the rate of material removal. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, in critical phases of the fabrication process the polishing endpoint cannot be determined merely as a function of polishing time. One way to determine the polishing progress is to remove the substrate and examine it at a metrology station. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. The state of the CMP progress may not be easy to observe or measure for some structures. This is particularly true of the CMP which is used as a photoresist lift-off aid during the fabrication of some type of magnetic sensors. Some methods for in-situ polishing endpoint detection monitor a parameter associated with the substrate surface, and detect an endpoint when the parameter abruptly changes. For example, if a dielectric layer is being polished to expose an underlying metal layer, the coefficient of friction and the reflectivity of the substrate will change abruptly when the metal layer is exposed. In the magnetic sensor case, this approach is not applicable.
The magnetic sensor for a magnetic head is deposited and initially patterned in a phase of the process which will be called “K3”. The K3 CMP process has been difficult to monitor due to the lack of necessary topography needed to measure the carbon recession which occurs in the CMP process. The carbon recession measurement is made later in the process at the “K5” stage where the hard-bias structures are formed at the sides of the sensor and the existence of surface topography makes measurement easier. Measurement at a subsequent phase is too late to use during the K3 CMP.
The magnetic sensor used in disk and tape drives can be any one of various types including tunnel-junction (TMR) and spin valves. In TMR and some spin valves designs the current in the sensor flows perpendicular to the film (CPP). The fabrication problems for TMR and CPP spin valves sensors are different than for those where current flows in the plane (CIP) of the film. FIG. 1 illustrates selected components in a TMR head 10 at a selected point during the fabrication process. The section is taken perpendicular the surface of the wafer. The set of layers comprising the sensor layer stack 12 are initially deposited over the entire wafer. A first layer of diamond-like carbon (DLC) 13 is deposited over the sensor layer stack 12. A photoresist mask 14 has been applied and the trench on the right side of the FIG. 1 has been formed. After the trench is formed a layer of alumina 15 is deposited followed by a second layer of DLC 16. The second layer of DLC 16 is typically significantly thicker than the first DLC layer 13. The wafer at the stage shown in FIG. 1 is ready for a chemical-mechanical polishing (CMP) process which is used to lift-off the photoresist 14 and the alumina and DLC films above it.
Scanning Electron Microscopes (SEMs) use an electron beam to image and measure features on a semiconductor wafer at higher resolution than optical microscopes. The electron beam causes secondary electrons and back-scattered electrons to be released from the wafer surface. Some SEMs can analyze the image using software to extract information. CD-SEMs are used in thin film manufacturing to measure the “critical dimension” (CD) of the sub-micron-sized features on a wafer to assure the accuracy of the process. The most advanced CD-SEM systems are fully automated and can process wafers without operator intervention. The system software can automatically detect features on the wafer that out of specification for further review and corrective action by process engineers. The contrast in CD-SEM images results from a variety of factors such as atomic number, density and dielectric constant of the materials.
U.S. Pat. No. 5,433,651 to Lustig, et al. describes in-situ chemical-mechanical polishing monitoring using a reflectance measurement is used to monitor the polishing process.
In published U.S. patent application 2004/0147048 by Jakatdar, Nickhil, et al., the invention includes an embodiment for designing underlying periodic calibration structures of varying line-to-space ratios in one or more underlying layers of a wafer for CMP monitoring. The periodicity of the underlying structure is positioned at an angle relative to the direction of periodicity of the target structure of the wafer.